From Dane Ortlund:

What is a Christian?
Not what do Christians believe, or how do they behave­—What is a Christian?
According to Jesus’ late-night talk with Nicodemus, a Christian is someone who has been born a second time—what theologians call regeneration. “Unless one is born again, he cannot see the kingdom of God” (John 3:3). “You must be born again” (John 3:7).

Created a second time

Jonathan Edwards explains what happens in the new birth:
"The believer has such a sight and such a knowledge of things that, ever since, he is quite another man than he was before. It has exceedingly altered his internal tempers and disposition. The knowledge that he has is so substantial, so inward, and so affecting, that it has quite transformed the soul and put a new nature into the man, has quite changed his very innermost principles, and has made things otherwise, even from the very foundation, so that all things have become new to them. Yes, he is a new creature, he is just as if he was not the same, but born again, created over a second time."

Niceness does not equal regeneration

It is fantastically easy to smooth over a rotten heart with niceness. The world is filled with nice people who have not been born again—nice, evil people. Niceness and wickedness are not mutually exclusive. They can even be mutually reinforcing (2 Tim 3:1-5).
A converted Christian, according to Edwards, is not the same person who is now really, really nice. They are not an improved version of the same person but a new person fundamentally re-wired. The new birth does not give us a new way to satisfy our old desires. It gives us new desires.
A believer does not differ from an unbeliever like an NFL linebacker differs from an All-American college linebacker—the same innate ability, only ratcheted up to the next level. A believer differs from an unbeliever as any football player differs from a corpse (Eph 2:1, 5). A true Christian has been granted resurrection, not refinement (Eph 2:6; Col 3:1).

A new direction, not perfection

Of course, all football players stumble on the field from time to time. Likewise, the regenerate stumble from time to time. Regeneration does not produce perfection; it inaugurates a new direction. While the effects of this radical change are worked out over a lifetime, the initial change itself is instantaneous, decisive, and permanent.
“The new birth does not give us a new way to satisfy our old desires. It gives us new desires.”
Christianity is not addition, but creation (2 Cor 4:6; 5:17). We can exhort a caterpillar to fly till we’re blue in the face but it will do no good until it is transformed into a new creature. And we can exhort a fallen human being to treasure Christ, love others, and shed niceness for real truth-in-love, but it does no good until that person is transformed and becomes a new creature.

I’m regenerated – so now what?

1. Spend time in hopeful prayer for others. God can bring life out of death in the heart of your next-door neighbor, just as he did for you.
2. Practice sober humility. Those born again did not invite God to bring them to life any more than Lazarus invited Jesus to bring him to life (John 11:43). Our faith and repentance are themselves gifts of grace.
3. Rejoice in calm assurance. Regeneration is irreversible. He started a good work in you, and he’ll see it through (Phil 1:6).

Adapted from A New Inner Relish: Christian Motivation in the Thought of Jonathan Edwards